Differential Protection

[speedtronic]

To check the stability of a Numeric/Digital differential relay for Generator, 0.8 A was applied from both sides (line side & neutral side) in all 3-phases at opposite direction. Differential current was zero as appeared on relay while restraint current at relay was 1.61A
Can anyone please explain this text to me. Its understandable that why differential current is zero but how restraint current is 1.6 A.

 

[Guest123]

Differential protection protects against short circuit faults on geneys, transformers etc.

The current on the ingoing and outgoing sides is monitored on both sides usually by current transformers (C.T's) and this value is what you get at your relay as restraint current.

Now the differential relay monitors this and accepts this as normal hence zero volts on the differential side however, should a short circuit occur and an imbalance be detected then you will have fault current on the differential side of the relay which will be different to the restraint current and cause the protective device to operate.

Also if an external fault travels through the system and becomes an internal fault the relay will pick this up and operate the protective.

This is the best I can offer off the top of my head, hope it makes sense.

Cheers.